1. Field of the Invention
The present invention relates to a method of controlling random access in a mobile communication system.
2. Description of the Related Art
Mobile communication systems based on code division multiple access (CDMA) employ a control process for starting packet transmissions from mobile stations. One such known control process, which is disclosed in Japanese laid-open patent publication No. 55693/97, for example, is made up of the following steps:
(1) First, a mobile station from which a packet transmission is to occur transmits a short burst, called a reservation packet, shorter than ordinary frame units, to a base station.
(2) When the base station receives the reservation packet, the base station permits the mobile station to transmit the packet frame which is reserved.
(3) Since the packet transmission is permitted, the mobile station transmits the packet frame.
The above control process is effective to increase the throughput of packet communications.
In a mobile communication system based on CDMA, as shown in FIG. 1 of the accompanying drawings, reservation packets for random access and information representing data packets that are continuous information and speech are transmitted and received within the same frequency band by code multiplexing. If the transmission power for reservation packets is large or reservation packets are concentrated to the extent beyond the communication capacity limit, then the level of interference within the same frequency band becomes higher than the allowable level for communications that use the frequency band. As a result, all communications that use the same frequency band fail to be received, interrupting the transmission and reception of continuous information such as speech information.
It is therefore an object of the present invention to provide a method of controlling random access in a mobile communication system based on CDMA to prevent continuous communications such as speech communications from being interrupted even when the load of random access communications varies.
In a method of controlling random access according to an aspect of the present invention, random access communications by mobile station for reserving an idle communication channel at a base station and continuous communications such as speech communications between the mobile and base stations are effected in different frequency bands or time slots respectively. Consequently, any effect which variations in the load of random access communications have on continuous communications such as speech communications can be eliminated or minimized.
Furthermore, in an embodiment of the present invention a mobile station lowers the spreading ratio and symbol rate for random access communications, thereby reducing the frequency band assigned for the random access communications. Consequently, the proportion of continuous communications such as speech communications can be increased.
In a method of controlling random access according to another aspect of the present invention, random access communications and continuous communications such as speech communications are effected in different frequency bands or time slots respectively, and the mobile station transmits a packet of continuous information such as speech information via a random access channel to the base station. When the reception of the packet through the random access communications is successful, the base station notifies the mobile station of the fact that the reception of the packet is successful. As a result, continuous communications such as speech communications are prevented from being interrupted.
The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.